1. Field of the Invention
The present invention relates generally to reference systems.
2. Description of the Related Art
Accurately-controlled currents are required in a number of modern electronic systems. For example, switched-current digital-to-analog converters (DACs) convert digital input signals to corresponding analog current signals with current sources which have corresponding switches that respond to the digital input signals. Because the accuracy of the current sources determines the conversion accuracy, they have generally been biased with reference signals that maintain substantially-constant currents despite parameter variations (e.g., base-emitter voltages) in the current-carrying devices (e.g., bipolar junction transistors).
Feedback-controlled circuits have been developed that realize effective steady-state current-control reference signals. Because these feedback circuits tend to have instability problems, however, they have sometimes been augmented with clamps which attempt to restrain signal movements until the feedback control stabilizes. Although the use of clamps indicates promise, it has generally been found that instability problems still persist in these feedback-controlled circuits.
The present invention is directed to stable current-control reference systems. Stability has been realized in these systems with sensors that shift clamp windows in response to a reference""s current-control signal to thereby maintain feedback control in the reference under steady-state and transient operating conditions.